1. Field of the Invention
The present invention is in the field of calcining systems for the manufacture of cement and the like and involves an independent heater means which cooperates with the conventional cooler in such systems to bring the cooler up to operating temperatures more quickly and more efficiently than occurs with conventional systems.
2. Description of the Prior Art
In the case of conventional calcining systems, it takes a significant time to reach temperature levels in the different zones of the calcining system which are appropriate to provide minimum temperature equilibrium. For example, to attain a suitable calcining temperature in a rotary kiln for the production of cement clinker, a temperature of recirculated air from the cooler must be in the range of about 650.degree. to 900.degree. C. before production can begin. Likewise, a calcinator requires an ignition temperature within the combustion chamber of at least 700.degree. C. Customarily, a cooler is used to provide recirculating gases, the gases being heated by the product being cooled. Since the cooler, whether of the grate, tube, satellite, or shaft type possesses relatively large quantities of material with correspondingly large heat capacities, this means that in the case of a cold cooler, with no hot product being passed therethrough, there is a considerable difficulty from the standpoint of losing time between the ignition of the burners for starting up the furnace and the initiation of production. It is not uncommon, for example, to require a time interval of 24 hours or more before production can start.
The difficulties in the case of a calcining system using a tube cooler are particularly severe. In the latter case, when the tube cooler is in the cold state, over a relatively long time interval the greatest portion of the introduced heat is consumed for the purpose of heating the mass of the cooler. Such coolers are characterized by relatively high radiation and convection losses before the cooler can transmit sufficient heat to the air being drawn into the system. Consequently, in a tube cooler utilizing cooling air having a relatively low speed which sweeps over a large surface of hot product there can be no recuperative air heating until all of the components such as the wall casing, the brick lining, and the internal fittings have attained an equilibrium temperature suitable for production.
The same problem is presented in the case of a satellite cooler which basically represents a special design of a tube cooler with a plurality of parallel cooling tubes.
If secondary and tertiary air flow through a cold tube cooler along its full length during the heating up period, the secondary or tertiary air is not available for calcination purposes for a long time.
It has already been proposed, in a somewhat different context, namely, for the purpose of economically integrating a cold-milling drying system with a calcining system, that during starting up of the calcining system, it is advisable to heat the air introduced from the milling system into the tertiary air conduit together with fuel. In this way, at least a portion of the hot furnace gases are circulated back into the milling system and the remaining portion of the furnace gases is used for the purpose of heating the tertiary air conduit. Thus, simultaneously with warming up the milling system, the tertiary air conduit is also heated up together with the air introduced from the milling system, and sufficient coal dust is blown into the tertiary air conduit as necessary for heating up the air to a level approximating the operating temperature of about 800.degree. C. Such a disclosure appears in German OS No. 29 31 214.